Examples of ring laser angular rate sensors are shown and described in U.S. Pat. No. 3,373,650 issued to J. Killpatrick and U.S. Pat. No. 3,390,606 issued to T. Podgorski, both of which are assigned to the assignee of the present invention. Such sensors include a pair of counter-propagating laser beams traveling along an optical closed-loop path including a plurality of mirrors. In order to obtain useful information, a small percentage of the counter-propagating laser beams is allowed to pass through one of the mirrors. The light beams are passed through a prism which combines the beams at slightly different angles and thereby creates a fringe pattern across a surface containing two photodetectors such as photosensitive diodes, each photodetector being much smaller than an individual fringe. Usually the photodetectors are separated by one quarter of a fringe spacing so as to provide readout output signals in phase quadrature. When the ring laser is rotated about its input axis, the counter-propagating beam frequencies change slightly, one increasing and the other decreasing. The difference in beam frequencies results in a beat frequency which is indicated by the rate of movement of the fringe pattern across the photodetectors. The photodetector outputs are fed into a logic circuit to count the number of fringes passing across the photodetector. The number of fringes passing each detector is directly related to the actual angular rotation of the sensor. The two detector output signals are compared to ascertain sensor rotation direction.
The prism optics of the readout determines the angle between the beams which directly forms the interference fringe pattern and the fringe spacing. In the prior art, the prism optics results in an angle between the beams in the order 25 arc seconds and produces only a single fringe. Two photodetectors are used which have photosensitive areas which are less than the width of a fringe spacing and the two photodetectors are separated by approximately one quarter width of a fringe spacing. To accurately achieve this arrangement, the angle between the beams which produce the fringe pattern must be held to tight tolerances.